Better error handling for channel opening under min

[thedon-chris]

Description

There is no proper error handling for attempting to open channel under required amount.

When attempting to open channel much under min, lightning-app just displays "Creating channel failed!".

This happened only when sending >20k sats, but when sending 20.5k sats another error showed.

Instead inform the user of minimum channel amount, and throw more informative errors.

Steps to reproduce the behavior

Attempt to open channel under 20k sats.
Attempt to open channel with 20.1-20.5k

Expected behavior

Throw informative error

Actual behavior

Does not throw informative error

Screenshot and/or logs if applicable

What it looks like now

What it should look like

[tanx]

Good call. We should definitely improve error messages in notifications.